What would help Democrats in 2020 more with regard to 2018: If the they retake at least the House or not?
Gaining one (or both) chambers in midterms of first-term presidents isn't necessarily helpful in the next presidential election. See 1954, 1994 or 2010.
It would be best to win the House in 2018 so they can actually start conducting some oversight on Trump. If over 4 years Trump has pulled shady stuff and done anything illegal or reckless, Democrats can make it into an issue using their power in the House. Now, you might say,
"Benghazi didn't stop Obama" - well, Benghazi was a tragedy mostly exploited and blown out of proportion by Republicans, and more focused on Clinton, and I think that is quite different than possible collusion with Russians to leak damaging material on his political opponent in the presidential election, or numerous conflicts of interest, and possible abuses of power.
As Chaffetz would say, Trump is target rich, and unlike Obama, he has
started his presidency deeply unpopular. He already has basically no room for error.
